Learn Russian: Этот актёр очень знаменитый.

Yes, you could say Этот актёр очень знаменит. However, using the full adjective (знаменитый) in this position is very common in everyday speech and treats his fame as a permanent, defining quality. The short form (знаменит) is also correct but can sometimes sound slightly more formal or bookish.

What is the difference between знаменитый and известный?

Both words translate to 'famous' or 'well-known', but they have a difference in scale. Известный simply means 'well-known' (from the root for knowledge or news). Знаменитый is stronger, meaning 'celebrated' or 'highly famous'. Since we are talking about a star actor, знаменитый is a perfect fit.

There is no verb in this sentence. Do we ever need a word for 'is'?

In the present tense, Russian almost always drops the verb 'to be'. You are literally saying 'This actor very famous.' However, if you change this to the past or future tense, you must include the verb: Этот актёр был очень знаменитый (This actor was very famous).

Does the letter ё in актёр tell us how to pronounce it?

Yes! A highly reliable rule in Russian is that if a word contains the letter ё, the stress always falls on that syllable. When you see it, you never have to guess where the emphasis goes.