Both have common interests regarding the effectiveness of Financial Internal Controls
Both have common interests regarding the compliance to financial reporting standards and the companies ordinance, and relevant laws.
Similarity of methods employed
Both assess risk, perform procedures, gain evidence and form an opinion based on the work done
Internal audit works for and on behalf of the organization whereas the external auditor is technically employed by and works for a third party, the shareholders.
But both of them seek active co-operation between the two functions. Also they carry out testing routines and this may involve examining and analyzing many transactions.