The diagram above explains why most employee engagement for sustainability fails. It is all set slap bang in the middle of the world of sustainability and takes into no consideration the interests, strengths and fears of the audience. The basic assumption is that if you bang on about sustainability loudly enough for long enough, the blue circle will shift leftwards sufficiently to meet your sustainability aims. Good luck with that.
My Green Jujitsu approach initiates engagement by finding the sweet spot of overlap between the sphere of interest of your audience and sustainability. If anything, it pulls the sustainability circle to the right until sufficient overlap can be found. So if your audience is engineers, you look for the engineering aspects of sustainability; if they are healthcare professionals, you look for where sustainability and health overlap (eg walking/cycling to work); if they are designers then you talk about cool eco-products and so on. As you proceed, the sweet spot will get larger as two circles converge.
I would take this a step further as you can see in the diagram below. If you look for the sweet spot from your sustainability side of the fence, you're more likely to misjudge where that audience boundary lies. If you put yourself in your audience's shoes and look at the problem from their perspective then you're far more likely to get it right.