What's more, the movie she's watching wasn't digitally retouched to insert Margot Robbie. The performance we're seeing is the actual Sharon Tate.
When watching the fight scene she mimes the choreography and flashes back to the rehearsals. Her trainer was Bruce Lee, and he's shown to be a supportive and effective teacher (in contrast to the blowhard jackass Cliff remembers him being).
Similarly, seeing Rick and Cliff just hanging out as friends as they watch Rick's episode of The F.B.I. together.
Rick bonding with Trudi Fraser is very sweet. Even more so when he becomes seriously concerned about looking unprofessional in front of her. When she excitedly tells Rick that his performance was the best acting she's ever seen. Rick is so touched he begins crying Tears of Joy, because only a little while ago, he was feeling depressed, worrying that he had lost his touch as an actor and was going to spend the rest of his career being a stepping stone for the new stars coming along. Trudi and the director praising his performance gave him a much-needed dose of self confidence and let him know that he's still got it.
After watching Rick's genuinely menacing performance in the show, culminating in him throwing Trudy's character roughly to the floor, it's quite sweet that the very first thing Rick does after the cameras stop rolling is lean forward and concernedly make sure that Trudy isn't hurt.
The film in general to some extent, being a fairly light (by Tarantino standards) character study of two Hollywood has-beens who ultimately are shown to be each other's best friends. The somewhat dark and extremely violent ending doesn't really dampen this, especially since it's still a significantly less dark ending than the actual Tate murders.
At the beginning, Rick talks to Cliff about the Polanskis living next door to him and how much he hopes he can meet them one day. At the end, it turns out the Polanskis were just as starstruck about having the great Rick Dalton living next to them and Sharon invites him inside for a drink after the confrontation with the Mansons.
It's also incredibly sweet that when Sharon meets Rick, she runs up to him and gives him a hug. Fridge Brilliance - not only is she a fan but she also knows he's just had a very hectic night and wants him to feel at ease. This is made even better by Sharon introducing him as a "wonderful actor" and excitedly asking her friends if they had seen Bounty Law.
While quickly played for Nightmare Fuel, Tex and Katie revealing to been fans of Bounty Law after realizing it was Rick Dalton yelling at them, with Tex even mentioning having a Bounty Law lunchbox as a kid. Overall, it shows that despite Rick's worries about being a has-been, many people fondly remember him and still see him as a star.
While largely a creepy scene, The Spahn Ranch does have a few of these. The most notable is Cliff insisting on checking on George and George appreciating the gesture despite not remembering Cliff, but even the Manson Family members get a couple. Theres a lot of hypocrisy towards their hostility towards Cliff, but they do all show concern when he beats up Clem, with one of the women mouthing to him that she loves him. And another riding to summon Tex to help. And while there is a lot of Paranoia Fuel to the idea of them giving guided tours to unsuspecting tourists, the girl giving the tour with Tex is seen chatting cordially with the tourist couple (although this is undercut by the fact that she took part in some of the subsequent murders in real-life).
After killing Sadie and realizing that his house was just broken into, Rick immediately starts screaming for Francesca to see if she's okay. While this has no bearing on their friendship, it's telling that she's the first person Rick instantly becomes concerned about rather than Cliff.
Meta example. During filming, Julia Butters, the young girl who played Trudi, became close with Leonardo DiCaprio to the point that when a group of men began dropping F-bombs, DiCaprio apologized to Julia and her mother, went over to the men and spoke with them for a while, and came back and said that no one would curse while Julia was around.
Another meta one: Sharon Tate's sister Debra was naturally very apprehensive when she first heard about the film, but after reading the script gave her wholehearted approval and even wept at the premiere because Margot Robbie's performance was so good she felt like she was actually seeing her sister again.
"She actually touched me in a way that convinced me that she was Sharon. She did such a damn good job that, for me, personally, the visit was a little short. I had Sharon back in front of me again, and it was too short a visit."
Debra also said that although the script was mostly finished when it was shown to her, Quentin Tarantino put in a few suggestions she had.
The simple fact that Sharon and her friends survived at the end of the film. For a few blissful minutes, the audience was allowed to exist in a timeline where Sharon, her unborn baby and her friends were safe and sound.