Sometimes, and sometimes not.
One of the major innovations from Marlon Brando and others in the 1940s-50s was to introduce a more "natural" performing style in both theatre and film.
Actors in theatre rarely have microphones, but audiences at the back of the theatre would still expect to hear their dialogue. As a result, words have to be well enunciated to be heard clearly. This level of enunciation is also typically more common amongst more "polished" upper-class people.
Theatre actors naturally moved into film, of course, but film does not require that type of enunciation in order for dialogue to be heard. Even if dialogue is recorded live, microphones are closer to the actors and are more sensitive, and dialogue is commonly re-recorded and overdubbed after filming anyway. Theatre actors still tended to follow the speaking patterns which they had learnt though. With film actors such as Brando and Eastwood becoming major stars, film actors started to use speaking patterns which worked better on film.
TV actors did not necessarily follow this though - TV programmes recorded on location did, but sitcoms were frequently filmed in front of a live audience (or wanted to appear as if they were), and actors there performed (and often still do perform) much more like stage actors. So in spite of technical reasons why it may not be required, TV actors often follow conventions of acting for a genre which are expected by audiences, even if those conventions make no sense.
And of course the same may apply to films where there are strong conventions for acting styles. Bollywood is one of these. People do not spontaneously break into dance routines in everyday life!
It's also important to remember that however bad an actor may be at speaking a language, on film they can record it over and over until they get it right. (Or right enough, anyway.) Like all Jackie Chan's films, the end of Rush Hour shows the "bloopers" where he got things wrong. Usually with Jackie Chan, this involves his stunts going wrong! On Rush Hour though, many of these involved Chan being taught how to say his lines correctly in English, a language he really doesn't speak. (And for balance, they also show Chris Rock taking a long time to get a sentence right in Cantonese.) So the script can have very sophisticated dialogue, even if the actors themselves would not be able to talk like that.