"János is a tall actor."
Translation:János magas színész.
Take a look at the Tips & Notes for the "Basic 2" lesson. It's explained there.
It this type of sentence: János is a tall actor, This is a big house, That is a red car, (subject) is a (adjective) (noun), most of the time, it is correct with egy, and also without egy. Using it is a matter of style or personal preference.
Especially with professions, the article can be dropped. János színész. is better than János egy színész. (while the second one is still acceptable).
In general, considering all kinds of sentences, having "egy" vs not having "egy" is a more complicated question.