The 25-year-old actress discussed the CW series and her friendship with Morgan, 27, during Monday's episode of A Little Late with Lilly Singh.
Petsch and Morgan play Cheryl Blossom and Toni Topaz on Riverdale. The characters started dating in Season 2 and share love scenes on the show.
Petsch said it's "amazing" to work with Morgan, her best friend, on set. She recalled how she first met Morgan when the actress initially auditioned to play Josie McCoy.
"We tested for Cheryl and Josie together," Petsch said. "We became best friends instantly."
"It's great, because I think when you have onscreen love interests ... it can be a little awkward and you don't really know each other, you're not comfortable," she added. "Vanessa and I are so comfortable that we'll write ourselves love scenes. We're like, 'Oh, we're not making out here? Let's make out.'"
Petsch was a bridesmaid at Morgan's wedding to MLB player Michael Kopech in Florida this month. Petsch shared a photo with Morgan on Instagram last week that was taken prior to the nuptials.
"I was lucky enough to be by my best friends side while she married one of the most genuine people I know. Still crying tbh. Love you V," Petsch captioned the post.
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Morgan posted a group picture the next day.
"My bridesmaids & brides-men, there are no words to describe how much I love you. Thankyou for making this day so special for me," she wrote.
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