Joaquin Phoenix was ''concerned'' his dog wouldn't get along with his girlfriend Rooney Mara's pooch.

The 43-year-old actor's beloved pet is always ''amazing'' with other people but he was worried about introducing her to his partner's own canine companion.

Thankfully, after some time, the two mutts eventually became ''best friends'' and now the couple love curling up with their pets together.

He said: ''I have a dog, and my girlfriend has a dog.

''They do [get along], and I was actually quite concerned about it because my dog is amazing with people, but sometimes doesn't do well with other dogs. It was a long process to get them [to co-exist].

''But now they're best friends. We were all curled up last night, and it was super sweet.''

The 'Mary Magdalene' actor - who has been dating Rooney, 32, since 2016 - has won a string of awards throughout his career, but he's always convinced someone else will do a better job with his roles than he can.

Discussing his thoughts as he draws closer to filming something new, he told Will Ferrell in a conversation for Interview magazine: ''I usually have a very strong desire to participate in an experience, and then shortly thereafter, I have an overwhelming desire to move as far away from it as possible. And then I try to get out of doing it.

''What usually happens is I start to think about other actors who would be so much better than me. 'I'm going to ruin this movie! This is a great director's great script, and I'm just going to f**k everything up. They should get someone else.' ''

Joaquin - who is vegan and known for his animal rights activism - had to ride a horse for his upcoming role in Western 'The Sisters Brothers', and he admitted he ''regretted'' having to mount the creatures because he ''felt bad'' for them, but he hadn't given much thought to that aspect of the production when he signed up.

He said: ''I didn't like riding horses. I actually regretted having to ride.

''I'm a little clueless, because I don't think that I really thought about it that much until I was there and realised, 'Oh, it's a Western and we're on horses a lot.'

''I hadn't really considered that. I felt bad about riding them. I had the feeling that they were like, 'I don't really want you on my back.' ''