Actor John Michie reveals Karl first kidnaps Stella in his taxi, then threatens to wipe Sunita's smug smile off her face and ends up sabotaging Jason's van.
He says Karl is a bitter and twisted man who will go to extreme lengths to get whatever he wants but Stella tells him to stay away.
"Karl is trying really hard to get back with Stella because he realises she is the love of his life," John said.
"Stella has absolutely no interest in Karl whatsoever, so he is struggling. He thinks he will always get her in the end because Karl thinks he will always get what he wants. It gets a little bit creepy but he needs to get her on her own so he can tell her how much he loves her," he explained.
"When people get obsessive about another character they don't realise how they're coming across and Karl doesn't really realise that he is coming across quite creepy."
You can expect a lot of drama from Karl Monroe. He is a man on the edge of a precipice. He's a desperate, desperate guy.
Karl ends up losing his job at Street Cars, although "he never blames himself".
But it is not just Stella who is on Karl's radar, as John reveals.
"Karl vows to have revenge on Jason and Sunita because they are the people that have put him in this rut that he is in, not himself," John said.
"He hates Jason, it is total jealousy. He is a younger man, he is a good looking guy, he is what Karl used to be and he cannot stand to see him with Stella."
"Karl starts to do some crazy stuff - it gets quite dark. He is a desperate, desperate man."
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