I had thought their going on strike wouldn't make much difference to movie/TV viewers, because the writers are already on strike. But there are unanticipated effects.
Actors cannot promote any of their work
While completed movies themselves will still premiere, no red carpets will be rolled out. In addition,
stars are not allowed to attend press junkets, press conferences, or even post about their work on social media, as that is considered publicity. Per their press conference on Thursday afternoon,
they are also not allowed to promote any work produced under the AMPTP banner, including past work. This ruling also extends to San Diego Comic-Con, scheduled for later this month, and any other conventions related to their work.
Actors are not allowed to take part in any panels and are encouraged to skip events completely.
This means no more chicken shop dates with Andrew Garfield, no more Tom Cruise surprising Mission: Impossible audiences in the theater, and no more Hot Ones.
While broadcast journalists, talk show hosts, soap opera stars, and some reality stars are members of SAG-AFTRA, they operate on a different contract from film actors and will therefore be unaffected by the strike (per Entertainment Weekly).