What is a 'cardboard character'?

[Olaf]

Business Idiom in English, Intermediate level

ESL/EFL Test #127 "Ways of commenting", question 2

It was quite a good film but I thought the characters were a bit like ..........

(a) paper
(b) cardboard
(c) plastic
(d) material

Business Idiom in English, Intermediate level

ESL/EFL Test #127 "Ways of commenting", answer 2

It was quite a good film but I thought the characters were a bit like cardboard.

Correct answer: (b) cardboard
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what is 'cardboard character'?

Olaf

[Conchita]

Characters that look unnatural or unreal can be defined as 'cardboard characters'.