MacLaine, 78, is set to appear as Lady Grantham's mother on the third series of the period drama , where Leech plays Tom Branson, an Irish chauffeur to the Crawley family who marries one of the daughters.
Speaking at the Monte Carlo Television Festival, Leech shared what it was like working with the iconic actress.
"Yeah, she's finished filming now but hopefully she'll come back," he said. "But she was absolutely amazing and just a life force. She arrived on set the first morning and what she was filming, you were just drawn into it.
"It's a living legend, you know, like Maggie [Smith] as well, and just the two of them talking... Shirley would turn and she'd go, 'When I opened for Sinatra...' and you go, 'OK...' You could write a book based on what was said!"
MacLaine is a six-time Academy Award nominee, whose career spans more than 50 years with memorable roles in The Apartment, Terms of Endearment and Steel Magnolias.
Leech added: "I don't know if it's a book she'd want written. It was all stories like that. I just found it fascinating.
"The Rat Pack in New York and LA and the stories of the parties that she'd go to and the people that would be there. And just the fact that her career spans so many... She'd tell stories and she'd mention Meryl Streep and then it's just amazing.
"I just found that fascinating. So maybe it's a book I should write for myself."
MacLaine was recently honoured with the American Film Institute's 40th Life Achievement Award.
Watch the Downton Abbey cast discuss series three at the National Television Awards below: