Costa faces a violent conduct charge from the Football Association for an alleged stamp on Liverpool’s Emre Can during Tuesday’s League Cup victory at Stamford Bridge.
Chelsea are contesting the FA’s decision, while Jose Mourinho, who defended Costa’s actions after the match, will not speak at his pre-match press conference on Friday ahead of his side’s clash with Manchester City.
When asked if Costa steps over the line with his conduct, Cahill told BBC’s Football Focus: "I think it’s his character. It’s they way he plays. Will you change him? Probably not, and I don’t think you should.
"I think, it’s certainly for me as a team-mate, he’s someone who you want in your team. It’s a strength he has, not just what you see how good he is with his feet and how skilful he is, for a big guy, but the way that he’s adapted to the Premier League. He’s a physical guy.
"I’ve played against him twice against when we played Atletico Madrid and I’ve played with him a lot this season. He’s a handful for any defender.
"One of his attributes as well, obviously his skills and scoring his fantastic goals, is that he puts his foot in and makes it difficult and uncomfortable for defenders."
