When I say recognisable I mean actors that so famous that when you watch a movie of theirs you think of them as the actor and not the character that they are playing. A Great example of this would be Will Smith. When I saw Ali it was like “oh look Will Smith is a boxer”. if you get an unknown for a part you cannot relate them to anything else apart from the part that you are watching them in. This makes the beleivability of their role easier.
As for Dan Aykroyd, Bill Murray and all the other actors in Ghostbusters. You may have recognised them from Saturday Night Live and some other smaller films such as Stripes and Blues Brothers but they were not known worldwide. Ghostbusters is what really made them recognisable worldwide and well established movie stars. In countries such as England where we had never even heard of Saturday Night Live never mind watched it, the chances areyou would have never even heard of Bill Murray or Dan Aykroyd. When I saw the movie in the cinema in 84 I didn't recognise one of the actors. This is one of the reasons the movie came across as beleivable.