In some countries (e.g., Guatemala), taking photos of people is considered improper and can raise some bad feelings. What is the custom in Cambodia of fotographing interesting people on the street?

It can be considered disrespectful like you are looking down on the Khmer as some kind of amusement. It doesn't hurt to ask permission to take someones picture. Even when not speaking the same language its easy to get the point across.
Khmer society is generally very conservative and should be treated with all due respect.

Thank you, Battanbanghelper. That is what I suspected from the little I know (only from reading) about Khmer culture. Thanks for taking time to advise me.

I think it is disrespectful to take anybody's picture in any country without first asking permission!
And in some countries, the potential subject will ask for a little money. Depending on the circumstances and my mood, I either give it and take the pic or walk away.

Don't pass on an opportunity to take photos of the Kmyer. You will quickly get a feel for when someone doesn't want their photo taken, but I have NEVER ONCE had someone object to a photo, unlike me when someone points a camera and hesitates and demands a smile.
Children love to see their photos and if you get to know them, give the camera to one of them and allow them to take pictures of their friends and family. The parents will likely bring their babies over for a photo as well. Something that is really appreciated is if you go to the photo lab and bring back an album of the pictures you just took. Copies from digital are very cheap (and same day service usually) and if you are in an area for any length of time, this is the best ice breaker when you don't speak each others language.
Some people may ask for a donations but I think that can be rude. You are just teaching them that everything has a price.

In Cambodia, senior citizens and Muslims generally don't like to be photographed even if asked politely. I get more headway with the senior citizens if I'm introduced as a friend of a friend/neighbour. Younger adults are agreeable if you ask for permission and (sometimes) explain why you want their picture. People in jobs might be hesitant about being photographed because they're afraid that the (innoucous) pictures may be used against them in some way e.g. temple ticket collectors sitting down could be accused of goofing off if the boss ever saw the photo.
People in the countryside are less suspicious of strangers and are more open to being photographed. Some are so happy that they overdo the posing and the resulting pictures look unnatural. Fruit vendors are also more amenable to being photographed if one makes a purchase.
Then you have ... little kids who wander into your frame as you're about to photograph a temple or something. You snap the picture because they smile a lot and they provide scale for the structure. The little tykes then ask for $1. This happens in urban areas with more tourist contact eg. Siem Reap.

The hardest thing you will have to deal with is people wanting to pose for you so they can see it. They love to see the picture after you have taken it. I've only had trouble once with taking pictures in all my travels, and that was when I was taking pictures of a police station in Myanmar. The beggers are happy if you take their picture, and give them a small amount of money after.

Your average Cambodian loves to be photgraphed, especially the children (love seeing the LCD display of your camera afterwards!!), adn they have great smiles. But as anyway be polite and get some form of consent first, this can be as simple as a smile and a nod showing your camera...
But be wary of taking photos of people in uniforms...