Alex Kingston happy to slap Doctor

Alex Kingston loved slapping Matt Smith while filming the new series of 'Doctor Who'.

The actress - who plays time travelling archaeologist River Song - got to hit and kiss Matt as The Doctor in scenes for the upcoming season and she admits she preferred hitting him to their smooch.

Alex said: "I had to slap him many times, and his cheek was getting redder and redder and he was actually getting irritated. He lost his sense of humour I think after slap five, I don't pull my punches. But the kiss was nice too. I didn't have to kiss him so many times as I had to slap him. But they were both quite fun."

The mysterious character has appeared in several episodes of the sci-fi series, and her true identity will finally be revealed during this year's episodes.

But Alex is one person who already knows who River is, and she was the only major cast member to be told the secret by showrunner Steven Moffat.

The 48-year-old star explained: "I do know who I am. I knew a lot before any of the others and I didn't reveal a thing."

Alex often gets approached on the street by fans who want to tell her their theories, which can be quite outlandish.

Referring to her encounters, she revealed: "Well I haven't had grandmother yet! I've had a lot of people actually say, 'Is she The Doctor from the future.' But it would be very weird. But I think The Doctor will always stay male, I can't imagine him as a woman, although I don't know what Steven has planned. But there's so much going on with her, can she be trusted? And is what she says true or false? You just don't know with her."

The new series of 'Doctor Who' premieres on BBC One on Saturday April 23.