how to intercut clips with lipsinking without losing sync?

Its explained in the iMovie help. Open iMovie help and do a search for "inserting video clips". The first item in the resulting list is the one you want. Its called "Inserting video clips or photos in a passage while maintaining running audio".

If you need more help than that, try posting additional questions here.

Hi Ramana - use the Advanced menu command 'Paste over at Playhead'. Firstly select and copy the clip to be superimposed, move the playhead to where you want it to start, and hit that command. The new clip will be inserted but the original video audio will be unaffected.

Hope that helps.

"... but the original video audio will be unaffected." This is only true if you have the iMovie preference "Extract audio when using 'Paste Over at Playhead' selected. I think this is the default setting so it shouldn't be a problem unless somebody changed it. Otherwise your original audio will be over-written.

Hi Thomas - yes, it is set by default, but you are quite right to point out that it could have been changed by the user. Anyway, hopefully that will solve his problem.
Cheers.