On the contrary, getting a "big name" actor to play a recurring role in a TV series is unusual. Having them be around for a once-per-season cameo or playing a role that only appears in a couple of episodes is much more realistic.
"Big name" typically means featured prominently on the movie poster - you don't see these actors on the small screen very often.
Samuel L. Jackson did appear in the 'Agents of Shield' TV show in his role as Nick Fury from the Avengers films - but *not* on a week-to-week basis. He appeared once for a cameo early in the first season, and I think in the season finale. I haven't watched later seasons of the show to know if they brought him back for more.
Checking out Natalie Portman's imdb page reveals that she has appeared on TV - in two episodes of the Simpsons, one episode of Sesame Street, and in the recently announced TV mini-series "We Are All Completely Beside Ourselves". That's it; the rest of her resume is feature films and shorts.
So, yes, it's unrealistic to cast her in the Silm Film project at all, but casting her in a minor role that won't recur is actually more likely to fly!
Luckily, we aren't constrained by reality and can cast whomever we like, regardless of whether or not they'd be caught dead on our show

.