what is difference between internal intercostal and external intercostal muscles

ORIGIN

The internal intercostals originate and insert between adjacent ribs.
The external intercostals originate on the inferior border of the rib above.

INSERTION

The external intercostals insert on the superior border of the rib below.

What actions do the internal and external intercostal muscles do?

The internal intercostals bring the ribs down during forced expiration.
The external intercostals elevate the ribs in inspiration.
Keep in mind that the abdominal muscles all have a role to play in forced exhalation.