Peggy became pregnant out of wedlock with Pete's baby in the first season, but hid the pregnancy from the married Pete. The eventual revelation of the pregnancy at first caused a further rift between Pete and Peggy, but ultimately led to a new friendship.
Speaking about the complicated relationship with IGN, Kartheiser expressed his belief that Pete and Peggy have gained respect for each other over time.
"I think time heals most wounds," he said. "I think a good example of where Pete sits on that is when she came to congratulate him on having a baby [with wife Trudy], and she said, 'Congratulations', and he immediately thought she was talking about business."
Kartheiser went on to suggest that Pete is one of the only Sterling Cooper Draper Pryce employees who truly values Peggy.
"I think he respects her as a copywriter. And he also respects her as someone who stays late and gets the job done, and that's what he does," the actor said. "Lots of people don't [pull their weight] in that office. Roger Sterling does not. The first couple seasons Ken Cosgrove really couldn't care less about his accounts.
"He was hardly working at all. So I think [Pete] sees in [Peggy] a partner in this extremely difficult job of getting some of the market share with this new agency."
Kartheiser recently said he's glad that Mad Men fans have warmed to Pete over the course of five seasons.
Mad Men airs on Sundays on AMC in the US and Tuesdays on Sky Atlantic in the UK.
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Watch a trailer for the fifth season of Mad Men below: