Physically the Celts are ...

Physically the Celts are terrifying in appearance, with deep sounding and very harsh voices. In conversation they use few words and speak in riddles, for the most part hinting at things and leaving a great deal to be understood. They frequently exaggerate with the aim of extolling themselves and diminishing the status of others. They are boasters and threateners and given to bombastic self-dramatization, and yet they are quick of mind and with good natural ability for learning.
 Diodorus Siculus

Quotes from the same author

It is absurd to entrust the defense of a country to people who own nothing in it.
 Diodorus Siculus
He (King Philip) wanted as many Greeks as possible to take part in the festivities in honour of the gods, and so planned brilliant musical contests and lavish banquets for his friends and guests. Out of all Greece he summoned his personal guest-friends and ordered the members of his court to bring along as many as they could of their acquaintances from abroad.
 Diodorus Siculus