Symbolism utalises colours, objects, animals, characters and sounds in order to incite emotion in the audience towards a particular issue or ideology. Though the symbol exists within a film – and might sway the audience’s opinion on a character or an event – it usually states something about the real world as
well – Kee from Children of Men (2006) is the last pregnant women alive, and she represents hope within the story… but she is also a representation of the innocence/hope in the real-world too, something that is being persued and must be protected (her being pregnant is also innocence).
A symbol can be strong and blaringly obvious, with lone shots that stand out but clearly have a deeper meaning (think about the dead monkey in Sunset Boulevard (1950), which symbolizes that despite Norma’s great wealth, her life is empty, or it can be more subtle, with something in the background that is so discrete that the audience hardly knows they’re being swayed – the milk in Clockwork Orange (1971)
showing that the characters are as helpless as a baby against the government/environment.
They can also be of varying degrees of directness – meaning that some symbols will be blaringly obvious as to their meaning, but some will require more intellectual thought. The balance is in that if the symbol is too cliché, the audience will roll their eyes, but if it’s too hidden, then they might not understand or see it. As an example, the filmmaker might want a child to see something that makes them loose their innocence. There are two options – the kid could be wearing white clothing, see the thing, fall over and get dirty – which would clearly symbolize loss of innocence – or she could be holding a balloon (maybe she’s at the circus) and when she sees the thing, she lets go and it floats off – her innocence floating away. While the clothing might be more obvious, it is 
also hackneyed and overdone, so the risk of it becoming cliché is there. That said, if the balloon doesn’t play off as it should, the audience might not even notice – and it has been wasted as well.
But while some symbols can be missed, others can be inferred even though they are not there. In Eva (1962), the writer goes to his room and sleeps with a girl. The fountain outside erupts, and many people took it that they were having sex – but this was not intended. Depending on what the symbol has implied and the context of the film, though, that might not be a big deal (lots of kids films have symbols for sex left right and centre), but if the symbol contradicted the message of the film – say that the Welsh writer is supposed to be gay, and it was a major point that he doesn’t have sex with women, and the fountain contradicted this – then the filmmaker may have some issues – whether it makes a character unclear, or celebrates something that should not be celebrated – something which more political films might have to be careful of.
As symbols generally need (or get) no verbal explanation, lots of experimental and non-narrative films contain symbolism. A pronounced example of this would almost any Kenneth Anger film, and his ability to pair images with sound in order to create contrast that invokes the audience to think… but are not self-explanatory enough to tell the audience what to think, but to make them judge for themselves.
References
Bakony, E 1974, ‘Non-Verbal Symbolism in Feature Films’, Journal Of The University Film Association, Vol. 26, No. 3, pp. 34-38
