Using 'diffident' might suggest weakness or lack of courage, so be careful with its implications. His diffident approach made others doubt his leadership ability.
Pro Tip 2/3
Subtle Flattery
'Diffident' can imply modesty in some contexts, which can be seen positively. Despite her success, she remained diffident about her accomplishments.
Pro Tip 3/3
Cultural Perception
In some cultures, being 'diffident' is valued more than being bold or outspoken. In the meeting, his diffident manner was appreciated by the company's Japanese partners.