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 FBI 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.
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 and hugs him. 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.
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 showing despite Rick's depression of being a hasbeen, many people fondly remember him and still see him as a star.
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 Tates 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 Robbies 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 space where Sharon, her unborn baby and her friends were safe and sound.