There's alot of things I wish happened on this film adaptation of Callan. The main one is casting. I would have loved to see Ronald Radd or William Squire reprise the role of Hunter and Anthony Valentine return as Toby Myres. Valentine didn't play the character during the original Aemchair Theater teleplay (Peter Boyles did) so this would have been new. The casting is still good as are the performances and if one is not so initiated with the tv series, would not mind these actors at all. Some familiar faces do return like Russell Hunter as Lonely (no one else can bring that role to life), and Clifford Rose as Snell, the Section's doctor with a chillingly indifferent aire about him.
This is a very small budget film but like the show, the strength is in the story and performances. The story which was the initial teleplay that started the serie is expanded to include some action sequences like Callan training to return to the service and a well shot action sequence involving an interesting car chase in the English countryside.
To me, this is about on par with the TV show, the difference is how James Mitchell expanded the story and not too much else. There is a difference with the movie from the TV show that can't be helped and that's how claustrophobic the TV show made everything seem. All these characters caught in this world of intrigue, very few places to hide. That added to the strength of the stories. Here, things are out in the open, it's all shot on film and that gives it some space. The viewer is not so drawn into it all.
If you don't mind limitations (this certainly doesn't have the budget of a Bond film) and like the genre, this film would be something you like. If you are a fan of a show but never got around to this, you might be a little disappointed for the reasons I mentioned but maybe not so much after a second or third viewing.