Sophie Turner Locks Lips with Costar Frank Dillane While Filming in Spain

Sophie Turner was lockin' lips with her costar, but it's not a new fling ... it's just another day at the office for her while the divorce from Joe Jonas moves forward.

The actress was spotted kissing "Fear the Walking Dead" actor Frank Dillane on a beach in Spain this week, while production continues on her British TV series, "Joan."

Frank is playing her husband on the project, and Sophie's portraying the lead character, Joan Hannington ... an infamous jewel thief who wreaked havoc in London back in the 1980s.

As we reported, Sophie popped up on set this week ... the first time she's been seen since the divorce news. She was taking a smoke break while filming another scene.

Joe filed divorce docs last week, asking for joint custody of their 2 young daughters.

Sources told us the breakup comes, in part, as a result of their different lifestyles -- and we were told Joe saw something on a Ring camera that made him realize it was over.