Use palm muting for breakdowns by lightly resting your palm against the strings while strumming, making sure the distortion is cranked up high. Palm muting gives a riff a sense of heaviness that many metal musicians describe as "brutal." Faster technical riffs can be palm muted too; this creates a staccato sound, whereas double picking without palm muting has a more open and loose sound. Double picking is, of course one of metal's key features and involves picking the strings up and down alternatively, in order to play faster. Some of the heaviest-sounding riffs use only a few notes and do not have to be made up of full chords. Listen to your favorite metal band and try and emulate the guitar playing when you practice.
Metal drummers are responsible for holding the music together and enhancing the sound qualities of the guitar riffs. For example, a drummer can make a palm-muted, double-picked riff sound heavy by using the crash symbol at a fraction of the tempo the guitar is playing. Increase the speed of a song by using "blast beats," in which the snare, cymbal and bass drum are all hit at the same tempo that notes are being played on the guitar. Keep songs interesting by adding breaks and pauses in which the guitar, bass or vocal is highlighted and the drums are halted long enough for a particular riff to be expressed before joining back in with the other instruments.
The vocal is where the particular genre of the song becomes especially important. Sometimes the style of vocals is what dictates the type of metal being played. For example, death metal singers often employ low, gurgling and guttural vocals, while black metal has more of a "shriek and scream" quality. Again, listen to bands whose style you wish to emulate and practice with the vocal qualities of that style in mind.
Bass players are the backbone of a metal song, often playing the same riffs as the guitar, with perfect timing. Use the lower octaves for heavy sections and breakdowns; initiate song sequences with brief solo riffs. Some metal players play bass with a pick (for faster riffs), while others use their fingers, as is traditional. Use whichever technique is most comfortable and allows for diversity in your playing.