Lionel can see why they call Prince Albert strapping, the way he struts about when he's feeling confident in his looks and abilities. When the suit jacket comes off and those suspenders make their appearance, Lionel knows that Bertie means business. His vowels are near perfect and his tongue twisters are flawless. When Lionel congratulates or encourages him, he hooks his thumbs in the straps, puts on a sheepish smile, and confidently begins again, usually with a bit more gusto at the praise.
Or when he's unsure with himself or is distracted during the lesson, he fiddles with his straps: loosening them, tightening them, moving them higher up his shoulders, or pushing them further down. His fingers itch to grab a cigarette, but he can't smoke in Lionel's office, so he plays with his suspenders instead.
Lionel's favorite thing about Bertie's suspenders, however, is when he's striding over to him, seductively slipping off each strap, followed by pants and shirt, and then they're kissing on the couch, lesson forgotten. When they can just be Lionel-the-friend and Bertie-the-man, nothing else matters. Nothing else matters but them.