Antonio is an outlaw in Illyria but also a sea captain who saves Sebastian from the storm after the shipwreck. The two become close friends because of the bond that is formed when people go through traumatic experiences together. Once Sebastian is healthy enough to go on his way into Illyria, Antonio wants to help him out by being a guide for him. The problem is that he isn't a friend to the Duke and must let Sebastian go on his own. Antonio rethinks this and decides to catch up to Sebastian to help him through Illyria by saying,
"But jealousy what might befall your travel,
Being skilless in these parts; which to a stranger,
Unguided and unfriended, often prove
Rough and unhospitable: my willing love,
The rather by these arguments of fear,
Set forth in your pursuit"(III.iii.8-13).
By this explanation, Antonio is a devoted friend and a person who would put his own safety on the line for someone else. Not much else is available as far as textual evidence to describe Antonio's history, likes, or dislikes. It is only in Act V that Antonio is able to plead his case to the authorities after being captured because he mistook Viola for Sebastian. Antonio explains that he was with Sebastian for three months after the shipwreck, which also explains the length of time they had to become close friends (V.i.92-94).
It would seem that Antonio is older than Sebastian and could possibly be the age of his father. Antonio seems to act more like a father-figure by seeking to protect Sebastian and by taking care of him for so long after the wreck. So, Antonio might be cast as one with a beard, wearing sea clothing, and possibly like eating fish. If one were asked to guess these things for an assignment, consider those three things and creating a picture of Antonio would be a little easier to do.