The camera path is only visible with using an alternate view or another camera view. You can use view options to turn the camera wireframe on all the time but there's no way to see the path keyframes without selecting the layer.
You could copy the camera position keyframes by selecting Position in the timeline and pressing copy, then create a new solid, make the solid 3d, start a mask by clicking anywhere on the solid with the pen tool, setting a mask keyframe, selecting the mask keyframe in the Timeline, then paste. This converts the camera position in x,y,z space to a mask path in x,y. Most camera moves are primarily in x,z so if you rotate the solid in x -90º and apply stroke you may end up with a line you can see in your comp that the camera will follow. If there's any camera movement in y the path won't be completely accurate, but it may be enough to help layout your elements.
I had thought of this approach, but I just don't know if it's me or AE that is giving me the problems.
I have a camera that is only animated XZ (there is no Y at all), but if I copy the cameras path and paste it into the mask shape parameter of the solid it loses all Z information. and all I get as a mask is a flat line.
Am I missing something??
You have a few problems copying a camera path to a solid mask. The first is that the camera position is based on the comp size so that motion outside of the comp boundaries is going to be off a solid that's big enough to cover the motion. The second is that you need to get the camera motion rotated 90º.
Rotating the camera path and then putting it back isn't much of a problem. Just add a 3D null, parent the camera to the null, rotate the null 90º in x, remove the parenting, copy the path keyframes, then parent the camera back to the null and reset the null x rotation to 0º.
If you use a solid and stroke then your path will be added in relation to the top left corner of the solid. If your solid is bigger than the comp, which would be my first guess at the way to do this, the position of the path won't match the camera position. If the solid is the same size as the comp the path could also end up way off the solid so a stroke wouldn't be visible.
To get around this use the pen tool to create a new shape layer. You only need 1 point but it's easier to see if you use 2. Set to stroke only. Now spin down the layer properties and find the path1, set a keyframe and paste. This will give you a path you can see no matter where it is in relation to the comp.
The shape anchor point is set at 0,0 by default so it needs to be fixed so the position of the path will match up. Fix it and rotate the shape -90º and you'll have what you want.
Find the Transform: Shape 1 property, change the anchor point from 0,0 to half your comp width and half the comp height. Then find the layer Transform x Rotation and rotate it -90º and you'll have a path that matches the camera that you can see.
Hope this helps.
Thanks Rick, that seems to have done it. Muchly appreciated on a Saturday too