Often when people say there's no emotion in the way a song is sung, it's usually because it's monotone and there's little variation. Ofcourse 'feeling' the lyrics helps you sing in a more interesting way, but often it's just a matter of learning great vocal technique that gives you freedom to colour the sound the way you want to. A good vocalist (and with good I mean someone that has practiced and learned a lot, I'm not talking about 'talent' here) can even make a song that he thinks has boring lyrics look like he's into it, not because he's actually into it, but because he has such freedom in his voice he can make his voice sound the way he thinks fits the song.
Work with dynamics, pronounce some words in a harder way. Also watch your twang, you can put more on certain words to make them stand out more too.
And you gotta play around with your lips and your tongue.
Just get different sounds in the same line, and with different sounds I don't mean different notes.
For example if I were too sing "youll never make me stay"
you could sing the 'youll' in your normal way, then at the 'never' you could sing a little bit harder and really open your mout, at 'make me stay' you could sing the stay a little longer and instead of just singing 'stay' the normal way you could up the twang a little bit.
I would recommend you to find the book Complete Vocal Technique by Cathrine Sadolin (if I'm not mistaken) and learn from that (and if you haven't already: get singing lessons), my vocal coach uses this book and we use the techniques in that book, which basically makes putting life and emotion into a sing not an issue anymore because the book puts the voice in different 'modes' you could utilize in a song and teaches you how to colour the sound the way you want it.