Grey's Anatomy has always boasted one of the most talented and impressive television casts. Over the years, the show has been able to compile together the likes of Ellen Pompeo, Sandra Oh, Patrick Dempsey, Katherine Heigl, Jesse Williams, and Kate Walsh all under one roof and it has amounted to 17 years of TV gold.

Related: The Behind The Scenes Drama Of Grey's Anatomy Used To Be Worse Than The Show's Storylines

This is such a terrific cast that it is totally hard to imagine anyone else in any of these roles, but unbeknownst to many diehard fans, some of their favorite iconic characters were close to being played by completely different actors. Here are some examples.

10 Rob Lowe

It's hard to imagine anyone other than Patrick Dempsey playing McDreamy, but ABC original wanted that part reserved for Rob Lowe, but the actor turned it down in favor of playing the title role in CBS's Dr. Vegas, which was cancelled after one season. Lowe elaborated on his decision in his memoir, where he admitted in a time most ABC shows flopped, he didn't expect much success for Grey's Anatomy.

Lowe further elaborated his regrets on Marc Maron's WTF podcast, saying that missing out on Grey's Anatomy probably cost him $70 million.

9 Joshua Jackson

In 2007, it was announced that Joshua Jackson would be playing opposite Sandra Oh as Dr. Cristina Yang's love interest, Dr. Owen Hunt. However, before he was able to step on to the set for the first time, the Hollywood Writers Guild of America went on strike, putting several shows on hold, including Grey's Anatomy.

Related: Interesting Facts About Joshua Jackson And Jodie Turner Smith's Relationship

When Grey's production finally came back underway in early 2008, Jackson was no longer available as he opted to star in FOX's Fringe instead. As a result, his part was recast by Kevin McKidd.

8 Amanda Foreman

Her name may not be familiar, but readers have seen her face on just about every classic show imaginable ranging from Felicity to Alias, and even the 2009 Star Trek movie. She almost nearly was able to add Grey's Anatomy to that resume.

When the show was in development, she went to audition for the part of Meredith Grey, but on the drive there, her agent called to tell her that Ellen Pompeo had already been casted. But since she was already on the way, Foreman showed up to auditions anyway, this time hoping to land the part of Cristina Yang, which she lost to Sandra Oh.

7 Sandra Oh As Miranda Bailey

Okay, these next few entries are going to be cheating, but they are also just too juicy and interesting to go unnoticed. Yes, Sandra Oh - among some other entries coming up next - was already a huge part of the Grey's Anatomy cast, but she was almost in a very different role.

Before she was cast as Cristina Yang, Sandra Oh was originally offered the part of Miranda Bailey, currently occupied by Chandra Wilson. However, Oh asked to play Cristina Yang instead and the rest, as they say, is history.

6 Isaiah Washington As McDreamy

Before Isaiah Washington was cast as Dr. Preston Burke, he auditioned for Dr. Derek Shepherd. On top of that, Shonda Rhimes was keen on casting Washington - or a Black man in general - in hopes of exploring interracial dating on television for Derek's relationship with Meredith.

Ultimately, producers were more sold on Patrick Dempsey as McDreamy, but still offered Washington a job as Burke. Suddenly, it makes sense why Dempsey and Washington were beefing once upon a time ago.

5 Jessica Capshaw As Nurse Rose

Jessica Capshaw's whole career was changed forever when she was cast as Dr. Arizona Robbins, but prior to her casting, she originally auditioned to play Nurse Rose, who was only booked for a couple of episodes in Season 5 as McDreamy's brief love interest. She lost the role to Lauren Stamile but ended up winning the part of Robbins.

Like Rose, Robbins was only booked for three episodes, but producers liked Capshaw so much that her contract was extended until the end of the season before she became a series regular in Season 6.

4 T.R. Knight Almost Didn't Audition

Before he ended up leaving the show, T.R. Knight's George O'Malley was one of our favorite characters. It is hard to imagine the show without Knight at the helm, but he almost never made it to the audition room, yet alone the hiring phase.

During an interview at the Paley Center for Media, he recalled having to travel from New York City to Los Angeles in order to audition for shows during pilot season and fell in love with the Grey's script. However, as an unknown actor, he struggled to get an audition booked for himself, but thankfully, he had an agent effortlessly fighting for him.

3 Camilla Luddington Missed Her Audition

At least T.R Knight eventually finally got into his audition. Camilla Luddington technically missed her chance to audition entirely. Luddington was told by her manager that Shonda Rhimes personally wanted Luddington to audition for Jo Wilson, but she missed it and producers didn't want to see anymore potential actors.

However, a few days later, Luddington's manager was then told that the producers still hadn't found their Jo Wilson and was willing to bring Luddington in later in the week. After auditioning in both her English and American accent, the producers decided Luddington was the perfect fit.

2 Justin Chambers Cast On A Whim

If we had to name any actor on the cast - or, in this case, who used to be on the cast - who may be the luckiest actor in Grey's Anatomy history, that honor may have to go to Justin Chambers, who was cast as Alex Karev only after the show finished filming its pilot episode.

Related: 15 Pics Of Justin Chambers That Make Us Miss Alex Karev Already

The producers realized in post-production that they needed another male character on board to help fill time. So, on a whim, they cast Chambers, filmed a couple of scenes with him, and edited them into the first episode. His tenure on the show wound up lasting over 15 years.

1 Ellen Pompeo Almost Turned Grey's Down

There could only ever be one Meredith Grey, and her name is Ellen Pompeo. At this point, it is practically impossible to imagine anyone else playing that role, but it is a role that Pompeo nearly turned down.

Previously, she auditioned for a show on ABC called Secret Service and had that show's pilot gotten picked up by a network, Pompeo would have taken that in favor over Grey's Anatomy. Thankfully, the show's pilot never got picked up and Shonda Rhimes was resilient in trying to get Pompeo on her own show until she said yes.

Next: Grey's Anatomy: These Facts Leave Fans Scratching Their Heads