If you want o debug every step of image creation using packer, you can use the -debug parameter.
For example,
packer build -debug app.json
This debug mode will enable you to select actions on you own.
One thing about debug mode is that parallelization is disabled for debug mode.
###Packer Logging
Also you can setup logging for your image builds. Set the following parameters from the terminal.
export PACKER_LOG_PATH="/path/to/packer.log"
export PACKER_LOG=10