The British comedian said he spotted her underwear hanging up and decided to have a feel so that when she wore them, he technically would have 'touched her boob space'.
Brand told FHM, 'I was at a costume fitting for Arthur and they had the bras hanging up that she'd be wearing and I thought, 'I'm going to go and touch them bras'.
'So I did and she was there and she saw me doing it, she just let me get away with it. They'd not been on her yet, but they will be. I touched her boob space.'
The 35-year-old, who is set to marry singer Katy Perry later this year, said he also got to share a bath with Ms Mirren for his role in The Tempest.
He added, 'I did The Tempest and that's where I first met her. She couldn't have been more charming, sexy, convivial, and brilliant.
'She just put me at ease. I couldn't have been more willing to have a particular oedipal bath with that woman.'