The other thing you can do to give your story depth and a three-dimensional quality is be symbolic.
Using symbolism is extending metaphor down to the ‘props’ of the performance, if you like. Last time we looked at C. S. Lewis’s children’s classic The Lion, the Witch and the Wardrobe in terms of how the very straightforward chase sequence of the plot — an evil witch pursuing children through a snowy forest — becomes resonant with depth when we see that there’s a symbolic reason why the wood is frozen and why it thaws as they go along. We also saw that the apparently common child characters — two brothers, two sisters, sibling rivalries and arguments and so forth — go deeper: all four children represent some of the Seven Character Archetypes which most readers subconsciously look for in fiction.
Now take a look at the objects that appear in the story, starting with the famous wardrobe. It begins as an ordinary piece of furniture but is of course charged with significance when it is revealed to be a portal to another universe. Edmund scoffs a typical childhood sweet, Turkish Delight, in his first meeting with the White Witch — but it represents temptation and leads him astray. The gifts given to three of the children by a returning Father Christmas (an obvious living symbol) represent abstract qualities of healing and fighting which they will demonstrate later in the tale; the Stone Table is a symbol of grim physicality and materialism, and its shattering later is metaphorical too.
You might think it’s cheating to find symbolism in a Christian allegory which is about magic but symbolism is rife in even the most material of stories. Harper Lee’s To Kill a Mockingbird is built around the bird in its title, in terms of its inner meaning; Graham Greene’s Brighton Rock uses the sea and Brighton itself as symbols of life and mystery; in Joseph Keller’s Catch-22, the character Milo comes up with the idea of selling chocolate-covered cotton to the government after he discovers that there is a glut of cotton in the market and that he cannot sell his own cotton -- the lack of substance beneath an enticing exterior is a theme in the novel. Similarly, in the same book, the soldier in white, a bandage-wrapped, faceless, nameless body that lies in the hospital in the first chapter of the novel, represents dehumanisation and the way that the army treats men as interchangeable objects.
Once you realise that it is symbols which give stories lasting meaning and power, you’ll start to see them everywhere, from the One Ring in The Lord of the Rings, to the pig’s head in Lord of the Flies, from the stage in Shakespeare’s As You Like It, to the watch in Quentin Tarantino’s Pulp Fiction. Of course, objects are objects and serve a material function within a story too — but the more symbolic resonance they can be given, the more the story itself will grow beyond its linear bounds and become a living thing for readers.