I've covered a few funerals before.
The thing is that you don't want to be caught smiling or asking people to smile. Be solemn, give respect to the deceased. Walk around slowly and quietly. Do not run, brisk walk or make lots of noise as it will irritate the family members. If you would like to take a picture of the deceased, ASK the direct family first. Make sure that they consult each other if not there might be squabbling within the family.
Some of the family members will choose to pose with the deceased. They would not be smiling (most of the time). So the best thing to say is "Ready, 1, 2, 3" rather than "1, 2, 3, Smile!"
Give due respect to the deceased, until the family members think that you are too stiff and need to relax. Personally I would not sit around if nothing is happening until the family tells me that it is okay.
Also, if the body is being transported to the crematorium, do encourage the family to take a picture with the deceased at the different parts of the crematorium, such as the back of the hearse, the hall for rituals, nearby the cremation room, and at the viewing room. Some family members are shy and would not want to take pictures at the crematorium, but it is the final stage of sending off and I believe if they have family overseas, they would want to see it there. (Some wanted to shoot at every single stage as most of the family is not local. Do ask if you are unsure.)
Unless they have specific instructions, the above should suffice you for a funeral.