Gwyneth Paltrow worried about being seen as an "evil stepmother".
The 50-year-old actress - who has Apple, 18, and Moses, 16, with first husband Chris Martin - admitted she had reservations about her relationship with now-husband Brad Falchuk's two kids Isabella, 18, and 16-year-old Brody because she was convinced whatever she did would be "wrong".
Speaking on a podcast for her brand Goop, Gwyneth said: "There's just no playbook for how to do it. I think there's this like archetypal evil stepmother and this inference it's going to be this fraught thing, so I came into it on tender hooks like, 'Oh my gosh, you can only kind of do the wrong thing.'"
The Iron Man star admitted her "trepidation" was her "one regret" about becoming a stepparent and finding her voice "shifted everything", so she'd advise other women in her position to treat their partner's children the same way they would their own.
She said: "However many years ago I was like, 'F*** it, these are my kids. I love them. I'm not gonna like be scared to discipline them.'
"If someone asked me for advice on it, I would just say from day one, just really treat them as your kid. I just wish I had done that earlier."
Brad - who has his teenagers with ex-wife Suzanne Bukinik - thinks Gwyneth is a "spectacular stepmom" and he loves the bond she shares with his kids.
He said: "You have a relationship with them outside of me. They talk to you all the time, they come to you for advice all the time, they rely on you."
In return, Gwyneth praised the TV producer as an "exceptionally good" stepfather and asked if he had a strategy in mind when they got married.
He replied: "I think you have to define yourself as a stepparent the way you would define yourself as a parent, like, 'Who am I in terms of archetype dad?'
"To me that just means providing stability and guidance and a sense of boundaries for the household and for the family. Also just trying to be a guide to them from my experience and and and and my wisdom."
Brad thinks Chris is a "great dad" and believes the Coldplay frontman is "a lot more fun" than he is.
He said: "The great thing is I don't have to be their dad. I don't have to provide the things their dad provides for them. He loves them and spends time with them and all that stuff. So, all I have to do is just be 'dad' when I'm the dad in the house."
The 51-year-old writer sees Apple and Moses in a "very sort of unique special way".
He added: "It's just coming in it with that confidence where I'm not trying to place their dad.
"But I am a dad to them and I've been lucky that there wasn't a huge amount of resistance to that. Fundamentally, I love them like they're mine, because they are."